The present invention relates to mechanical power supplies and actuators. It finds particular application in conjunction with high force, low travel extensible actuators and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in conjunction with other high pressure fluid systems, as well as other mechanical power supplies, pumps, motors, valve controllers, and the like.
Heretofore, numerous techniques have been utilized for converting thermal expansion into mechanical power. The large power levels generated by thermal expansion have been converted to useful work in heat engines and other devices. Most commonly, the energy of a liquid/vapor phase thermal expansion is harnessed to capitalize on the ready transportability of liquids. The transportability facilitates a mass transfer of heat for effecting phase change.
One way to effect the mass transfer is by physically moving the liquid to a heat source, then allowing the liquid/vapor to flow against resistance to a cooling source. Steam and other types of external combustion engines typify this technique. In another technique, fuel liquids or liquid aerosols are pumped into a combustion chamber. The mix is ignited and the combustion vapor is permitted to exit the chamber against resistance, physically removing the heat and clearing the chamber for the next cycle. The internal combustion engine typifies this mode. These two modes utilize the well-known capabilities of mass transfer as an efficient method of transporting heat and minimize the need for thermal conduction for moving heat.
These liquid/vapor phase techniques have several drawbacks. First, the vapor phase is compressed at very high pressures. An unanticipated release of these pressures creates shock waves associated with a blast, hurling debris in a dangerous manner. The material is transported and expended, requiring complex controls and valving, as well as a continuous supply of liquid to replace the liquid lost or consumed.
Rather than transporting the material which is acted upon by the heat to expand and contract, the heat itself may be moved. More specifically, heat can be conducted into and out of a sealed chamber which expands during the heating cycle and contracts during the cooling cycle. The sealed chamber technique has many advantages including its mechanical simplicity, proportional control, high stiffness actuation, ready adaption to a variety of heat sources, high power density, and silent operation. Moreover, because liquids compress only a small amount, as compared to vapors, they tend to be much safer than a liquid/vapor system. Unfortunately, the transfer of heat into and out of the medium normally relies on thermal conductivity. Mediums which exhibit good expansion/contraction ratios upon melting have relatively poor thermal conductivities. Thus, a solid/liquid sealed chamber phase change actuator tends to have a very slow cycle time.
The present invention contemplates a new and improved solid/liquid or solid/solid phase change sealed chamber actuator which can exhibit a reduced cycle time on the order of the cycle time of a solenoid, i.e. about one half second or less.